CHAMBERS, J.

¶1 — Lamtec Corporation, based in New Jersey, manufactures insulation and vapor barriers. It sells its products nationwide and did more than $1.1 million in business in Washington State each year during the seven years at issue here. Lamtec has no offices or agents permanently in Washington but regularly sends representatives to visit customers. In 2004, the Department of Revenue (Department) determined that Lamtec's Washington sales were subject to business and occupation (BO) tax. Lamtec argues that under the federal commerce clause it had an insufficient nexus to Washington to be subject to the State's BO tax. Lamtec paid under protest and then filed a refund claim in superior court. The trial court dismissed Lamtec's action, and the Court of Appeals affirmed. Lamtec Corp. v. Dep't of Revenue, 151 Wn. App. 451, 215 P.3d 968 (2009). We affirm the Court of Appeals.

FACTS

¶2 Lamtec manufactures its products at its facility in New Jersey and has no permanent facilities, office, address, phone number, or employees in Washington. It sells its products wholesale to customers who place orders by tele-phone. Washington customers ordered over $9 million worth of Lamtec's products from 1997 to 2003. About two or three times a year during the tax period at issue, three Lamtec sales employees visited major customers in Washington. During those visits, the employees did not solicit sales directly, but they answered questions and provided information about Lamtec products. The trial court found that approximately 50-70 such visits occurred during the period at issue, and the purpose of these visits was to maintain Lamtec's Washington market.

¶3 In May 2004, the Department requested a statement from Lamtec regarding its Washington business activities. Based on the company's response, the Department required Lamtec to register and submit a Master Business License Application. The company's application listed its estimated gross annual income in the state as "$100,001 and above." Clerk's Papers (CP) at 427. The Department then assessed $45,599.76 in tax, $15,959.94 in penalties, and $9,996.42 in interest. Lamtec unsuccessfully petitioned the Department for a correction. Lamtec paid the tax under protest and challenged the tax in Thurston County Superior Court. Both Lamtec and the Department moved for summary judgment. The court granted the Department's motion, finding a substantial nexus between the taxed activities and Washington, and it concluded that the commerce clause does not prevent Washington from imposing the tax. Lamtec unsuccessfully appealed to the Court of Appeals on several issues and petitioned for review only on the substantial nexus question. We granted review. Lamtec Corp. v. Dep't of Revenue, 168 Wn.2d 1009, 226 P.3d 782 (2010).

STANDARDS OF REVIEW

¶4 This case raises questions of law on appeal from summary judgment. Our review is de novo. Dreiling v. Jain, 151 Wn.2d 900, 908, 93 P.3d 861 (2004) (citing Rivett v. City of Tacoma, 123 Wn.2d 573, 578, 870 P.2d 299 (1994)). We interpret statutes so as to implement the legislature's intent. Ski Acres, Inc. v. Kittitas County, 118 Wn.2d 852, 856, 827 P.2d 1000 (1992) (citing In re Bale, 63 Wn.2d 83, 86, 385 P.2d 545 (1963)). When its meaning is in doubt, a tax statute "must be construed most strongly against the taxing power and in favor of the taxpayer." Id. at 857 (citing City of Puyallup v. Pac. Nw. Bell Tel. Co., 98 Wn.2d 443, 448, 656 P.2d 1035 (1982)). Taxes are presumed valid. However, while we interpret statutes to give effect to legislative intent and review summary judgments de novo, the taxpayers have the burden of proving they are factually exempt. RCW 82.32.180 ("At trial, the burden shall rest upon the taxpayer to prove that the tax as paid by the taxpayer is incorrect, either in whole or in part, and to establish the correct amount of the tax."); Gen. Motors Corp. v. Washington, 377 U.S. 436, 441, 84 S. Ct. 1564, 12 L. Ed. 2d 430 (1964) ("'a taxpayer claiming immunity from a tax has the burden of establishing his exemption'" (quoting Norton Co. v. Dep't of Revenue, 340 U.S. 534, 537, 71 S. Ct. 377, 95 L. Ed. 517 (1951))).

ANALYSIS

¶5 Washington imposes a gross receipts tax (BO tax) "for the act or privilege of engaging in business activities" on "every person that has a substantial nexus with this state." Former RCW 82.04.220 (1961); see also Ford Motor Co. v. City of Seattle, 160 Wn.2d 32, 39, 156 P.3d 185 (2007). A tax on an out-of-state corporation must satisfy both the requirements of the due process clause of the Fourteenth Amendment and the commerce clause. Quill Corp. v. North Dakota, 504 U.S. 298, 305, 112 S. Ct. 1904, 119 L. Ed. 2d 91 (1992)). The due process inquiry considers whether the corporation has sufficient contacts with the taxing state such that imposing the tax "does not offend 'traditional notions of fair play and substantial justice.'" Int'l Shoe Co. v. Washington, 326 U.S. 310, 316, 66 S. Ct. 154, 90 L. Ed. 95 (1945) (quoting Milliken v. Meyer, 311 U.S. 457, 463, 61 S. Ct. 339, 85 L. Ed. 278 (1940)). Although Lamtec calls our attention to a case where the Court of Appeals struck down a BO tax assessment on due process grounds, it does not allege such a violation here. Pet'r's Suppl. Br. at 11-16 (citing City of Tacoma v. Fiberchem, Inc., 44 Wn. App. 538, 722 P.2d 1357 (1986)).

¶6 Instead, Lamtec argues that the assessment violates the "negative" or "dormant" commerce clause. The dormant commerce clause "prevents state regulation of interstate commercial activity even when Congress has not acted . . . to regulate that activity" but does not "relieve those engaged in interstate commerce from their just share of state tax burden." BLACK'S LAW DICTIONARY 305 (9th ed. 2009); W. Live Stock v. Bureau of Revenue, 303 U.S. 250, 254, 58 S. Ct. 546, 82 L. Ed. 823 (1938). Under modern dormant commerce clause jurisprudence, in order for a state to tax an out-of-state corporation, the tax must be (1) "applied to an activity with a substantial nexus with the taxing State," (2) "fairly apportioned," (3) nondiscriminatory with respect to interstate commerce, and (4) "fairly related to the services provided by the State." Complete Auto Transit, Inc. v. Brady, 430 U.S. 274, 279, 97 S. Ct. 1076, 51 L. Ed. 2d 326 (1977); see also Ford, 160 Wn.2d at 48-49. Lamtec disputes only whether it has a "substantial nexus" with Washington State.

¶7 Lamtec argues that an entity has sufficient nexus with Washington for purposes of the BO tax only if it has a "physical presence" here and contends that it does not have such a presence. Pet'r's Suppl. Br. at 3 (citing Nat'l Bellas Hess, Inc. v. Dep't of Revenue, 386 U.S. 753, 758, 87 S. Ct. 1389, 18 L. Ed. 2d 505 (1967)); Quill, 504 U.S. at 309. The Department suggests that this case is not a good vehicle for considering whether physical presence is required because, in its view, Lamtec clearly maintains such a presence and, alternatively, that the physical presence requirement is limited to sales and use taxes and does not apply to the BO tax. Instead, in the Department's view, a business is subject to Washington's BO tax if "'the activities performed in this state on behalf of the taxpayer are significantly associated with the taxpayer's ability to establish and maintain a market in this state for the sales.'" Resp'ts' Suppl. Br. at 5 (quoting Tyler Pipe Indus., Inc. v. Wash. State Dep't of Revenue, 483 U.S. 232, 250, 107 S. Ct. 2810, 97 L. Ed. 2d 199 (1987)).

¶8 We note that Lamtec and the Department each interprets "physical presence" differently. Lamtec contends that the physical presence test requires a "small sales force, plant, or office" in the taxing state. Quill, 504 U.S. at 315. Lamtec suggests a "brick and mortar" presence or at least an established sales force within the taxing state is required to establish the requisite nexus. Lamtec effectively urges us to adopt the "bright-line" physical presence test required for sales and use taxes established by Bellas Hess, 386 U.S. at 758, in the mail order context. The Department concedes the company does not have a brick and mortar presence but argues that under Tyler Pipe, 483 U.S. at 250, significantly less activity within a state is sufficient to establish a nexus for BO taxes.

¶9 The United States Supreme Court has made clear that an established sales force is sufficient to satisfy the nexus requirement. It has not held that an established sales force (or a physical presence) is a requirement to establish the requisite nexus. "Whether or not a State may compel a vendor to collect a sales or use tax may turn on the presence in the taxing State of a small sales force, plant, or office." Quill, 504 U.S. at 315 (emphasis added) (citing Nat'l Geographic Soc'y v. Cal. Bd. of Equalization, 430 U.S. 551, 97 S. Ct. 1386, 51 L. Ed. 2d 631 (1977) (finding the presence of two small offices sufficient for imposition of a duty to collect sales and use tax even though the activities conducted, soliciting advertising, did not relate directly to the taxed sales)). In National Geographic, 430 U.S. at 556, the United States Supreme Court reserved judgment on California's "slightest presence" rule, finding the society's continuous presence "sufficient" for nexus. This language does not establish a "requirement."

¶10 Similarly, Lamtec suggests that the United States Supreme Court required "continuous local solicitation" to establish substantial nexus. Scripto, Inc. v. Carson, 362 U.S. 207, 211, 80 S. Ct. 619, 4 L. Ed. 2d 660 (1960) (finding the presence of 10 independent contractors sufficient for nexus). The court in the case cited did in fact base its finding of substantial nexus in part on continuous local solicitation, but does not call it a requirement. Id. Lamtec further asserts that, in a similar case, the United States Supreme Court "found that the crucial factor supporting Washington's jurisdiction to impose [BO] taxes was that the sales representatives' activities, allowing the taxpayer to establish and maintain a market, actually took place in Washington." Pet'r's Suppl. Br. at 16-17 (citing Tyler Pipe, 483 U.S. at 250-51).

¶11 The Department draws our attention to a number of cases where courts found sufficient presence for substantial nexus based on contacts with the taxing jurisdiction that are similar to those here. E.g., Standard Pressed Steel Co. v. Dep't of Revenue, 419 U.S. 560, 562, 95 S. Ct. 706, 42 L. Ed. 2d 719 (1975) (taxpayer's denial of substantial nexus "verges on the frivolous" even though its only continuous presence in the state was one employee who did not solicit or accept orders); Tyler Pipe, 483 U.S. at 249-51 (finding substantial nexus where wholesaler's "solicitation of business in Washington is directed by executives who maintain their offices out-of-state and by an independent contractor located in Seattle"); Orvis Co. v. Tax Appeals Tribunal of N.Y., 86 N.Y.2d 165, 180-81, 654 N.E.2d 954, 630 N.Y.S.2d 680 (1995) (finding sufficient physical presence based only on 41 service visits over 3 years). The Department also cites a Washington Board of Tax Appeals opinion finding sufficient presence for purposes of the BO tax based on even less significant contacts. Carr Lane Mfg. Co. v. Dep't of Revenue, No. 54917, 2001 WL 718027, 2001 Wash. Tax LEXIS 270 (Wash. Bd. Tax Appeals Jan. 22, 2001). We find these authorities persuasive. A physical presence in the taxing jurisdiction for purposes of BO tax can be based on periodic visits.

¶12 We note that the United States Supreme Court itself has cast some doubt on the reach of the physical presence test it established in the sales and use context. The Quill Court stated that the case establishing the physical presence requirement, Bellas Hess, might have been decided differently under contemporary commerce clause jurisprudence and upheld it in the sales and use context largely due to stare decisis and the fact that the mail order industry had relied on it as a "bright-line." Quill, 504 U.S. at 311, 316-17. The Quill Court's main reason for upholding the physical presence requirement for sales and use taxes in the mail-order context was to "firmly establish [] the boundaries of legitimate state authority to impose a duty to collect sales and use taxes and reduce [] litigation concerning those taxes." Id. at 315. The requirement suggested by Lamtec that the presence be "continuous" or "constant" would, on the contrary, create considerable uncertainty. Take, for example, a company that relies on nonexclusive independent contractors who are continuously located in the taxing state, but who devote only a small or slight amount of time to the company's projects, or a company with a sales staff that spends a considerable amount of time in the taxing state but is based in a neighboring state. Establishing physical presence in such cases would likely depend on the time and activities conducted within the taxing state.

¶13 As New York's high court pointed out in a case almost indistinguishable from this one,

acceptance of the thesis[ — ]that Quill made the substantial nexus prong of the Complete Auto test an in-State substantial physical presence requirement — would destroy the bright-line rule the Supreme Court in Quill thought it was preserving in declining completely to overrule Bellas Hess. Inevitably, a substantial physical presence test would require a "case-by-case evaluation of the actual burdens imposed" on the individual vendor involving a weighing of factors such as number of local visits, size of local sales offices, intensity of direct solicitations, etc., rather than the clear-cut line of demarcation the Supreme Court sought to keep intact by its decision in Quill. Thus, ironically, the interpretation of Quill urged by the vendors here would undermine the principal justification the Supreme Court advanced for its decision in that case, the need to provide certainty in application of the standard and with it, repose from controversy and litigation for taxing States and the nearly $200 billion-a-year mail-order industry, with respect to sales and use taxes on interstate transactions.

Orvis, 86 N.Y.2d at 177 (citation omitted) (quoting Quill, 504 U.S. at 315).

¶14 There is also extensive language in Quill that suggests the physical presence requirement should be restricted to sales and use taxes. See, e.g., Quill, 504 U.S. at 314 ("[W]e have not, in our review of other types of taxes, articulated the same physical-presence requirement . . . established for sales and use taxes."). Many of our sister courts have refused to apply the physical presence test to other kinds of taxes. E.g., Geoffrey, Inc. v. Comm'r, 453 Mass. 17, 899 N.E.2d 87, cert. denied, 129 S. Ct. 2853 (2009); Lanco, Inc. v. Dir., Div. of Taxation, 188 N.J. 380, 908 A.2d 176 (2006); Tax Comm'r v. MBNA Am. Bank, N.A., 220 W.Va. 163, 640 S.E.2d 226 (2006). However, authorities are not unanimous: the Tennessee Court of Appeals has found "no basis for concluding that the analysis" should be different for franchise and excise taxes than for sales and use taxes, although it acknowledged that the Quill Court "expressed some reservations" about the requirement. J.C. Penney Nat'l Bank v. Johnson, 19 S.W.3d 831, 839 (Tenn. Ct. App. 1999). But, as reviewed above, the great weight of authority concurs with the Department.

¶15 Even if a brick and mortar physical presence or substantial sales force is not required under due process and the dormant commerce clause, Lamtec urges us to adopt such a standard as a matter of policy for clarity sake. There is some appeal to a bright-line test for business taxation. However, we have already largely rejected Lamtec's invitation. We addressed a similar issue in Tyler Pipe Industries v. Department of Revenue, 105 Wn.2d 318, 715 P.2d 123 (1986), vacated in part, 483 U.S. 232. Tyler Pipe had its principal place of business in Tyler, Texas, and distributed cast iron, pressure and plastic pipe, and fittings nationwide. Id. at 320. Tyler Pipe did not have a place of business or employees within Washington but utilized independent contractors to perform the function of sales representatives. These agents performed activities within Washington necessary to maintain a market for Tyler Pipe. Id. at 320-21. We approved the Department's stated requisite minimal connection of "nexus" in former WAC 458-20-193B (1970), "the crucial factor governing nexus is whether the activities performed in this state on behalf of the taxpayer are significantly associated with the taxpayer's ability to establish and maintain a market in this state for the sales." Tyler Pipe, 105 Wn.2d at 323. We concluded that Tyler Pipe had a substantial enough relationship with Washington State to satisfy the nexus requirement, despite the fact that it had "no personnel designated as employees residing in Washington." Id. at 321, 327. Instead of employees, Tyler Pipe used independent contractors to represent its interests. Id. at 324. We found the difference between employees and independent contractors to be "without constitutional significance." Id. (citing Scripto, 362 U.S. at 211). Instead, we looked to the actual activities "by the in-state sales representative which helped Tyler Pipe establish and maintain its market in this state." Id. We found the activities were substantial and affirmed the State's authority to impose the tax. Id. at 327. Although it reversed on other grounds, the United States Supreme Court affirmed our holding that there was an adequate nexus to support Washington's jurisdiction to tax.

As the Washington Supreme Court determined, "the crucial factor governing nexus is whether the activities performed in this state on behalf of the taxpayer are significantly associated with the taxpayer's ability to establish and maintain a market in this state for the sales." The court found this standard was satisfied because Tyler's "sales representatives perform any local activities necessary for maintenance of Tyler Pipe's market and protection of its interests. . . ." We agree that the activities of Tyler's sales representatives adequately support the State's jurisdiction to impose its wholesale tax on Tyler.

Tyler Pipe, 483 U.S. at 250-51 (citation omitted) (quoting Tyler Pipe, 105 Wn.2d at 321, 323). We agree with the Department that the "crucial factor" in this language is that the activities were "significantly associated with the taxpayer's ability to establish and maintain" its market.

¶16 We conclude that to the extent there is a physical presence requirement, it can be satisfied by the presence of activities within the state. It does not require a "presence" in the sense of having a brick and mortar address within the state. We do not see a material difference whether the activities are performed by a staff permanently employed within the state, by independent agents contracted to perform the activity within the state, or persons who travel into the state from without. The activities must be substantial and must be associated with the company's ability to establish and maintain the company's market within the state. The contacts by Lamtec's sales representatives were designed to maintain its relationships with its customers and to maintain its market within Washington State. Nor were the activities slight or incidental to some other purpose or activity. We hold that Lamtec's practice of sending sales representatives to meet with its customers within Washington was significantly associated with its ability to establish and maintain its market.

CONCLUSION

¶17 A BO tax is a tax on conducting business within the state. Several requirements must be met under the commerce clause before a state may levy such a tax on an out-of-state business. Among other things, there must be a substantial nexus between the taxing state and the activity taxed. Complete Auto, 430 U.S. at 279. We find that this case is largely controlled by our decision in Tyler Pipe, 105 Wn.2d 318. Although Lamtec did not have a permanent presence within the state, by regularly sending sales representatives into the state to maintain its market, Lamtec satisfied the nexus requirement. We affirm the Court of Appeals and hold that the Department had authority under the commerce clause to impose a BO tax.

MADSEN, C.J., and C. JOHNSON, OWENS, FAIRHURST, and STEPHENS, JJ., concur.